The invention relates to a valve with an AND gate function in the case of which a valve member able to be shifted in relation to a valve housing controls the fluid connection between two inlets and an outlet, its switching position being dependent on the pressure difference between the fluid pressure obtaining at the two inlets so that at equal input pressures at least one inlet is joined with the outlet, and the valve member can assume two opposite terminal positions in which it clears one of two first passages stationary arranged in the housing and connected with the outlet and simultaneously shuts off the other first passage.
A valve having these features is disclosed in the product catalog entitled “Der Pneumatic-Katalog 97/98”, issue of 97/98, thirty-third edition, Festo AG & Co., page 11.5/10-2. This valve possesses one outlet and two inlets, the outlet always being subject to pressure, when both inlets are simultaneously supplied with a fluid pressure. In the case of identical pressures the switching position is indifferent and the outlet is connected only with one or simultaneously with both inlets. If at the inlet there is a pressure difference, then only the lower pressure will appear at the outlet.
The same product catalog mentions an OR gate valve on page 11.5/10-1, which also possesses two inlets and one outlet. In this case higher of the two inlet pressures always arrives at the outlet. The inlet supplied with the lower pressure is shut off.
The German patent publication DE 22 51 963 A discloses a logical valve gate with an inlet and two outlets. In this case the pressure medium flowing in through the permanently open inlet is alternatingly let off through the two outlets, a change over always taking place when the input pressure returns to zero or is reduced by a predetermined amount.
In safety related fluid power circuits, where safety plays a substantial role, there may be a requirement to supply a load simultaneously at all times from two pressure source and to shut off the supply for reasons of safety, when one of the two pressures falls below a certain threshold, for example because of a leak occurring. There is consequently a requirement for a fluid power circuit, which unlike the above mentioned known AND valve, separates a load completely from the pressure supply, when on the inlet side a certain pressure difference occurs.